Covering element for covering a slot between two adjacent tooth tips of an electromagnetic machine

ABSTRACT

Provided is a covering element for covering a slot between two adjacent teeth of an electromagnetic machine, a stator for an electromagnetic machine and a method of forming a covering element. The covering element comprises a first sub-element and a second sub-element. The first sub-element and the second sub-element are arranged adjacent to each other in a first direction. Further, the second sub-element is made of a different material than the first sub-element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European application No. EP17191123.3 having a filing date of Sep. 14, 2017 the entire contentsboth of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to the field of flux linkage between the rotor andthe stator of an electromagnetic machine. Particularly, the presentinvention relates to a covering element for covering a slot between twoadjacent tooth tips of an electromagnetic machine and a method offorming a covering element.

BACKGROUND

In today's electromagnetic machines a radially enlarged tooth tip isprovided at each tooth of a stator for reducing a poor flux linkage of awide slot opening between adjacent teeth of the stator.

As shown in FIG. 8, a stator 800 comprises a first tooth 861 and asecond tooth 862 with a slot 865 in-between. A coil 870 is inserted inthe slot 865. Each tooth 861, 862 has a symmetric tooth tip. Each toothtip comprises two grooved portions 881. Between the grooved portions 881a conventional covering element 890 made of a non-magnetic material, isinserted for closing a gap between the grooved portions 881 and forinsulating the grooved portion 881 from the other grooved portion 881.The gap is i.a. necessary for inserting the coil 870 into the slot 865.

In some current solutions, stator teeth are provided withcircumferential enlarged tooth tips for ameliorating a flux linkagebetween the rotor and the stator. However, an insertion of the statorcoil in the slot between adjacent teeth may be difficult because of theshape of the radially enlarged tooth tips.

Hence, a damage of the inserted coil may be the consequence.Particularly, the turn-turn insulation as well as the main wallinsulation may be damaged during the insertion. Additionally, thesedamages of the turn-turn insulation and/or the main wall insulation maynot be found during a production acceptance testing. However, thesedamages may reduce the insulation lifetime and hence the lifetime of theelectromagnetic machine.

The lifetime of an electromagnetic machine is crucial from the economicperspective. Further, the reliability of the electromagnetic machine isa key factor for the operational costs.

SUMMARY

An aspect relates to a reliable and simple covering element for anelectromagnetic machine which is able to enhance a flux linkage in theelectromagnetic machine.

According to a first aspect of embodiments of the present invention acovering element for covering a slot between two adjacent tooth tips ofan electromagnetic machine. The covering element comprises a firstsub-element and a second sub-element. The first sub-element and thesecond sub-element are arranged adjacent to each other in a firstdirection. The second sub-element is made of a different material thanthe first sub-element.

The tooth tip is arranged at a radially inner end or a radially outerend of a stator tooth. Whether the tooth tip is arranged on the radiallyinner end or the radially outer end depends on the stator used in theelectromagnetic machine. The covering element is arranged between thetip regions of two adjacent teeth of the electromagnetic machine suchthat the covering element covers the slot in-between the two adjacentteeth. Hence, the covering element may secure a stator coil insertedinto the slot, inside the slot and at the same time may link themagnetic flux from the rotor to the stator.

The sub-element is a portion of the covering element. Each sub-elementmay be formed individually and may then be linked/fixed together. Thelinkage of the different sub-elements may be a mechanical linkage,moulding or a chemical linkage. For example, the second sub-element maybe glued with a two component epoxy glue, as for example Araldite, tothe first sub-element. According to another exemplary embodiment, thefirst sub-element and the second sub-element get bonded together with anexpandable resin laminate during a vacuum pressure impregnation process.

The first direction of the covering element may correspond to thecircumferential direction of the electromagnetic machine, i.e. thecircumferential direction of the stator. Hence, the sequentialarrangement in the first direction, when the covering element is mountedbetween one tooth and another tooth, is the one tooth, the firstsub-element, the second sub-element and the other tooth.

Advantageously, a ripple in an air gap flux caused by a change inreluctance due to a conventional slot may be reduced. Furthermore, thecovering element according to embodiments of the invention may reduce acogging torque and a torque ripple. Additionally, the covering elementis easily mounted to the stator teeth.

Additionally, the covering element may increase the performance of theelectromagnetic machine by creating lower losses and thereby shorterpayback time.

According to a further exemplary embodiment of the present invention,the covering element further comprises a third sub-element. The firstsub-element, the second sub-element and the third sub-element arearranged adjacent to each other in the first direction such that thesecond sub-element is arranged in-between the first sub-element and thethird sub-element. Further, the second sub-element is made of adifferent material than the first sub-element and the third sub-element.

The second sub-element may be glued with a two component epoxy glue, asfor example Araldite, to the first sub-element and the thirdsub-element. According to another exemplary embodiment, the firstsub-element, the second sub-element and the third sub-element get bondedtogether with an expandable resin laminate during a vacuum pressureimpregnation process.

According to a further exemplary embodiment of the present invention,the covering element further comprises a fourth sub-element. The fourthsub-element is arranged in-between the first sub-element and the secondsub-element or in-between the third sub-element and the secondsub-element.

The fourth sub-element is made of a magnetic material, a soft magneticmaterial or a non-magnetic material.

According to a further exemplary embodiment of the present invention,the first sub-element and the third sub-element are configured for beingeach coupled to a tip region of a tooth.

The coupling of the first sub-element and the third sub-element,respectively, to the tooth tip is made by a press fit. A first sidesurface seen in the first direction of the first sub-element is pressfitted to a flank of a first tooth tip. Further, a second side surfaceof the third sub-element seen in the first direction is press-fitted toa flank of a second tooth tip. The first tooth tip and the second toothtip are adjacent tooth tips. The first side surface and the second sidesurface are located on opposing sides of the covering element seen inthe first direction.

The flux linkage between the rotor and the tooth tips is directed as thefirst sub-element and the third sub-element are made of another materialthan the second sub-element. Additionally, the entire slot between twoadjacent tooth tips is covered with the covering element. Hence, no airgaps are formed between the tip regions of the teeth.

By coupling each of the first sub-element and the third sub-element to atooth tip, a slot ripple in an air gap flux caused by changes in areluctance due to the slot may be reduced because a flux linkage may beprovided by the first sub-element and the third sub-element,respectively, such that the magnetic flux do have a better flux path.Therefore, the efficiency of the electromagnetic machine may beimproved.

Additionally, an open slot between the two tooth tips can be formedwider than in a conventional stator. Hence, a coil may be insertedwithout using a special inserting tool. Hence, the coil may not bedamaged during insertion and the lifetime of the insulation may not becompromised and hence increased.

Furthermore, when the covering element comprises the first sub-elementand the second sub-element, the first sub-element and the secondsub-element are configured for being each coupled to a tip region of thetooth.

According to a further exemplary embodiment of the present invention, afirst magnetic permeability of the first sub-element is higher than asecond magnetic permeability of the second sub-element.

According to a further exemplary embodiment of the present invention, athird magnetic permeability of the third sub-element is higher than thesecond magnetic permeability of the second sub-element.

The magnetic permeability is a dimension for the ability of a materialto foster the formation of a magnetic field within itself

The first sub-element's magnetic permeability and the thirdsub-element's magnetic permeability can be higher than the secondsub-element's magnetic permeability. The first magnetic permeability canbe equal to the third magnetic permeability. Hence, the magnetic fluxlinkage to the two different adjacent tooth tips will be equallydistributed.

On the one hand, when the magnetic permeability is low, there is no fluxlinkage or only a low flux linkage over the second sub-element. Hence,the two adjacent tooth tips are decoupled with respect to the magneticflux. On the other hand, when the magnetic permeability is high, theflux leakage between the two neighbouring stator teeth is high.

By providing the first sub-element and the third sub-element,respectively, with a higher magnetic permeability than the secondsub-element, at the same time a good flux linkage between the rotor andthe stator tooth, via the first sub-element and the third sub-element,without a flux leakage between two adjacent tooth tips over the secondsub-element may be formed and maintained. Hence, a better flux linkagemay be achieved and thereby an increased efficiency.

The covering element can be made as a single part that has differentproperties in different areas. As an example, the covering element couldbe made from SMC powder where different types of powder with specificpermeability is placed in a die in areas where the specific permeabilityis required, after the pressing and heat treatment the covering elementhas different permeabilities in different areas.

As an alternative to SMC stainless steel powder with differentpermeability (like austinitic, ferritic, etc. can also be used) placingthe different powder materials in the die in the areas where thespecific properties are required. After pressing the material issintered. This covering element has a very high mechanical robustness.

The width of the areas with different properties can be varied to anywidth by moving a separation in the filling-equipment in the pressingmachine, and thereby give different properties to the covering element.

According to a further exemplary embodiment of the present invention,the second sub-element is made of a non-magnetic material or a softmagnetic material. Further, the first sub-element and the thirdsub-element are made of a magnetic material or a soft magnetic material.

Furthermore, when the covering element comprises the first sub-elementand the second sub-element, the second sub-element is made of anon-magnetic material or a soft magnetic material, and the firstsub-element is made of a magnetic material or a soft magnetic material.

When the first sub-element and the second sub-element or the firstsub-element, the second sub-element and the third sub-element are eachmade of a sot magnetic material, the magnetic permeability of the secondsub-element is lower than the magnetic permeability of the firstsub-element and/or the third sub-element.

The non-magnetic material is for example a fiber glass laminate.

Additionally, a length of the second sub-element in the first directionbeing of a non-magnetic material and the lengths of the firstsub-element and the third sub-element in the first direction being of amagnetic material may be individually varied based on the neededperformance and/or magnetic linkage. Particularly, the length and/or theshape of the first sub-element and the third sub-element may beoptimized based on electromagnetic performance calculation.

The first sub-element and the third sub-element may be made of twodifferent magnetic materials for influencing the respective amount ofmagnetic flux through the first sub-element and the third sub-element,respectively.

Furthermore, the non-magnetic second sub-element may reduce magneticflux losses between two adjacent stator teeth compared to a completemagnetic covering element.

According to a further exemplary embodiment of the present invention,the first sub-element comprises a first coupling section and the thirdsub-element comprises a second coupling section. Furthermore, the firstcoupling section and the second coupling section are each coupleable ina respective groove of the two tooth tips.

The coupling section is a portion of the respective sub-element at whichthe sub-element is in contact with one flank of a stator tooth.

The first coupling section and the second coupling section may each havea shape which is axis-symmetric to a plane extending in the radialdirection of the stator and being at a middle position between the firstcoupling section and the second coupling section.

The groove which is formed in the flank of the tooth, has a shape whichcorresponds to the shape of the first coupling section and the secondcoupling section, respectively.

The first coupling section and the second coupling section may bepress-fitted to the respective groove. Hence, the covering element isinserted between two adjacent stator teeth with a high force and may notfall out due to operational agitations or vibrations.

According to a further exemplary embodiment of the present invention,the first coupling section and the second coupling section are eachformed as a protrusion in the shape of a V, a W, a semi-circle, asemi-ellipse, a semi-drop, a polygon, a triangle and/or a rectangle.

The covering element further comprises a third direction which extendsperpendicular to the first direction. The third direction may beparallel to the rotational axis of the electromagnetic machine in whichthe covering element is integrated.

The coupling section being in the shape of in the present applicationmeans that the respective outer side surface of the sub-element facingthe stator tooth, seen in a cross-sectional view having a normal beingparallel to the second direction, has a defined shape. By defining thatshape, the transmittable force as well as the holding force between thesub-element and the respective stator tooth is increased due to anincreased friction between the coupling section and the groove. Hence,the stability of the covering element in the stator is increased.Therefore, the covering element may not inadvertently fall out the slotbetween the respective stator teeth.

According to a further exemplary embodiment of the present invention, asum of a first length of the first sub-element in the first direction, asecond length of the second sub-element in the first direction and athird length of the third sub-element in the first direction is a totallength in the first direction. Further, a middle of a length of thesecond sub-element in the first direction coincides with a middle of thetotal length in the first direction.

The length of the sub-element is defined as the maximal length of thesub-element in the first direction. Particularly, when the couplingsection of the sub-element is formed as a protrusion in a V-shape, thelength is measured from the point of inflexion of the V.

Particularly, the first coupling section is formed identical to a firstcoupling surface between the first sub-element and the secondsub-element and the second coupling section is formed identical to asecond coupling surface between the third sub-element and the secondsub-element. Further, the first coupling surface, the second couplingsurface, the first coupling section and the second coupling section areall formed as protrusions in a V shape. Then, the first length ismeasured from the point of inflexion of the V of the first couplingsection to the point of inflexion of the V of the first couplingsurface. Similarly, the third length is then measured from the point ofinflexion of the V of the second coupling section to the point ofinflexion of the V of the second coupling surface.

The total length is measured from the outermost position of the firstcoupling section in the first direction to the outermost position of thesecond coupling section in the first direction. For example, when thefirst coupling section and the second coupling section are both formedas a protrusion in the shape of a V, the total length is measured fromthe point of inflexion of the one V to the point of inflexion of theother V.

The middle of the second length of the second sub-element in the firstdirection coincides with the middle of the total length in the firstdirection in this application describes that the covering element isformed such that the second sub-element is arranged in the middle of thecovering element. Preferably, when the second sub-element isaxis-symmetric to the middle of the second length, the covering elementis axis-symmetric to the middle of the total length. In this exemplaryembodiment, the second sub-element is arranged in the middle of thecovering element. Therefore, the flux linkage between the firstsub-element and a stator tooth is equal to the flux linkage between thethird sub-element and a further stator tooth.

According to a further exemplary embodiment of the present invention, asum of a first length of the first sub-element in the first direction, asecond length of the second sub-element in the first direction and athird length of the third sub-element in the first direction is a totallength in the first direction. Furthermore, a middle of the secondlength of the second sub-element in the first direction is distanced inthe first direction from a middle of the total length in the firstdirection.

According to a further exemplary embodiment of the present invention,the first sub-element, the second sub-element and the third sub-elementare formed as an integral part.

Furthermore, when the covering element comprises the first sub-elementand the second sub-element, the first sub-element and the secondsub-element are formed as an integral part.

Forming the first sub-element, the second sub-element and the thirdsub-element may be performed by pressing and heat treatment of two ormore powder materials with different magnetic properties.

By forming the covering element as an integral part, the stabilityagainst wear and material fatigue at the interfaces between thedifferent sub-elements of the covering element may be enhanced. Hence,the life-time of the covering element may be increased.

By forming the magnetic first sub-element and third sub-element,respectively, by a vacuum pressure impregnation process together withthe non-magnetic second sub-element, stress peaks due to materialborders may be omitted.

According to a further aspect of embodiments of the present invention, astator for an electromagnetic machine is disclosed. The stator comprisesat least two teeth, and at least one covering element as describedabove. Each of the at least two teeth comprises a tooth tip with agroove. The at least one covering element is arranged in-between the atleast two tooth tips. The first sub-element comprises a first couplingsection and the third sub-element comprises the second coupling section.Further, the first coupling section and the second coupling section areeach coupled in the respective groove of the two tooth tips.

Further, when the covering element comprises solely the firstsub-element and the second sub-element, the first sub-element comprisesthe first coupling section and the second sub-element comprises thesecond coupling section.

The groove in the tooth tip is formed as the negative to the couplingsection formed as a protrusion. The covering element is press-fittedbetween the two tooth tips. Hence, the covering element may ensure thespatial arrangement of the coil in the slot between two adjacent teeth.

Furthermore, the slot in-between two adjacent teeth may not be narrowedby a radially enlarged tooth tip. Therefore, an easy insertion of thecoil without a semi-closed or asymmetric tooth tip, may be ensured.

Hence, no damage of the coil insulation during insertion occurs.Additionally, no special coil tool must be used. Instead, a simplerobotic arm may be used for the insertion.

According to a further exemplary embodiment of the present invention,the respective groove has a shape being complementary to a shape of afirst coupling section or a second coupling section.

The shape of the groove is the negative of the shape of the couplingsection, wherein the covering element is press-fitted to the statorteeth. Hence, the holding force is increased due to a surface contactextending over the entire surface of the coupling section.

The shape of the slot and the coupling section may depend for example onmanufacturability of the shape as well as on frictional aspects.

According to a further aspect of embodiments of the present invention, amethod of forming a covering element is described. The method comprisesproviding a first sub-element and providing a second sub-element.Further, arranging the first sub-element and the second sub-elementadjacent to each other in the first direction, wherein the secondsub-element is made of a different material than the first sub-element.

According to a further exemplary embodiment of the present invention,the first sub-element and the second sub-element are made as oneintegral part.

According to a further exemplary embodiment of the present invention,the first sub-element and the second sub-element are made by pressingand heat treatment of a powder with different magnetic properties.

Furthermore, when the method comprises providing a first sub-element,providing a second sub-element and providing a third sub-element,arranging the first sub-element, the second sub-element and the thirdsub-element adjacent to each other in the first direction such that thesecond sub-element is arranged in-between the first sub-element and thethird sub-element, wherein the second sub-element is made of a differentmaterial than the first sub-element and a third sub-element, then thefirst sub-element, the second sub-element and the third sub-element aremade by pressing and heat treatment of a powder with different magneticproperties.

It has to be noted that embodiments of the invention have been describedwith reference to different subject-matters. In particular, someembodiments have been described with reference to apparatus type claimswhereas other embodiments have been described with reference to methodtype claims. However, a person skilled in the art will gather from theabove and the following description that, unless otherwise notified, inaddition to any combination of features belonging to one type ofsubject-matter also any combination between features relating todifferent subject-matters, in particular between features of theapparatus type claims and features of the method type claims isconsidered as to be disclosed with this application.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with references tothe following Figures, wherein like designations denote like members,wherein:

FIG. 1 shows a covering element according to an exemplary embodiment ofthe present invention;

FIG. 2 shows a cross-section of a covering element according to anexemplary embodiment of the present invention made from powder pressedtogether and heat treated;

FIG. 3 shows a cross section of a covering element according to anexemplary embodiment of the present invention made from powder pressedtogether and heat treated;

FIG. 4 shows a covering element according to an exemplary embodiment ofthe present invention;

FIG. 5 shows a stator and a covering element according to an exemplaryembodiment of the present invention;

FIG. 6 shows a stator with an inserted coil before mounting the coveringelement according to an exemplary embodiment of the present invention;

FIG. 7 shows the stator of FIG. 6 after mounting the covering elementaccording to an exemplary embodiment of the present invention; and

FIG. 8 shows a conventional stator with an inserted conventionalcovering element according to the prior art.

DETAILED DESCRIPTION

The illustrations in the drawings are schematically. It is noted that indifferent figures, similar or identical elements are provided with thesame reference signs.

FIG. 1 shows a covering element 100 according to an exemplary embodimentof the invention. The covering element 100 comprises a first sub-element110, a second sub-element 120 and a third sub-element 130. The firstsub-element 110, the second sub-element 120 and the third sub-element130 are arranged adjacent to each other in a first direction 181. Thesecond sub-element 120 is made from a different material than the firstsub-element 110 and the third sub-element 130.

A second direction 182 extends perpendicular to the first direction 181and a third direction 183 extends perpendicular to the first direction181 and the second direction 182. A first cross-sectional surface 140 ofthe covering element 100 comprises a first normal 141 being parallel tothe third direction 183. The first cross-sectional surface 140 comprisesa first outer surface 151 which extends parallel to the first direction181, a second outer surface 152 which extends parallel to the firstdirection 181 and is distanced from the first outer surface 151.Additionally, the first outer surface 151 and the second outer surface152 are interconnected by a first coupling section 153 and a secondcoupling section 154, respectively. Further, the first coupling section153 is distanced from the second coupling section 154.

As depicted in FIG. 1, the first coupling section 153 and the secondcoupling section 154 are each a protrusion in the shape of a V.Additionally, the first coupling section 153 and the second couplingsection 154 are axis-symmetric to a middle axis 185 of the coveringelement 100. The middle axis 185 is parallel to the third direction 183and is positioned in a middle of the total length of the coveringelement 100 in the first direction 181.

The first sub-element 110 comprises a first coupling surface 121 beingan interface to the second sub-element 120 and distanced from the firstcoupling section 153 in the first direction 181. Similarly, the thirdsub-element 130 comprises a second coupling surface 122 being aninterface to the second sub-element 120 and distanced from the secondcoupling section 154 in the first direction. The second sub-element 120comprises a third coupling surface 123 being an interface to the firstsub-element 110, and a fourth coupling surface 124 being an interface tothe third sub-element 130. The third coupling surface 123 and the fourthcoupling surface 124 are distanced from each other in the firstdirection 181.

Additionally, the third coupling surface 123 is formed corresponding tothe first coupling surface 121 and the fourth coupling surface 124 isformed corresponding to the second coupling surface 122. Particularly,when the first coupling surface 121 is formed as a groove in the firstsub-element 110, the third coupling surface 123 is formed as acorresponding protrusion in the second sub-element. Accordingly, whenthe fourth coupling surface 124 is formed as a groove in the thirdsub-element 130, the fourth coupling surface 124 is formed as acorresponding protrusion in the second sub-element 120.

As depicted in FIG. 1, the first coupling surface 121 is formed as aV-shaped groove in the first sub-element 110 and the second couplingsurface 122 is formed as a V-shaped groove in the third sub-element 130.Accordingly, the third coupling surface 123 and the fourth couplingsurface 124 are formed as corresponding V-shaped protrusions.

The first coupling surface 121, the second coupling surface 122, thethird coupling surface 123 and/or the fourth coupling surface 124 may beformed as a groove or as a protrusion, for example in the shape of a V,a W, a semi-circle, a semi-ellipse, a triangle, a semi-drop, a polygon,and/or a rectangle.

FIG. 2 shows a cross-section of a covering element 200 according to anexemplary embodiment of the invention. The normal of the cross-sectionis the first normal 141 (shown in FIG. 1) which is parallel to the thirddirection 183.

A first sub-element 210, a second sub-element 220 and a thirdsub-element 230 are arranged one adjacent to another in the firstdirection 181. The second sub-element 220 is arranged in-between thefirst sub-element 210 and the third sub-element 230 in the firstdirection 181. A first length 213 is the length of the first sub-element210 in the first direction 181. A second length 223 is the length of thesecond sub-element 220 and a third length 233 is the length of the thirdsub-element 230, each in the first direction 181. The first sub-elementcomprises a first coupling section 253 which is formed different fromthe first coupling section 153 (shown in FIG. 1). The first couplingsection 253 is in the shape of a triangle. A second coupling section 254is formed as an axis-symmetric triangle as well. The first couplingsection 253 and the second coupling section 254 are axis-symmetric to amiddle axis 285. A first coupling surface 221 and a second couplingsurface 222 are each formed straight, i.e. parallel to the seconddirection 182. Hence, the first coupling surface 221 and the firstcoupling section 253 are of different shape. Furthermore, the secondcoupling surface 222 and the second coupling section 254 are also ofdifferent shape.

The first length 213 in the first direction 181 is measured from thefirst coupling surface 221 to the point of inflexion of the firstcoupling section 253, which is the point of inflexion of the triangle.In the same manner, the third length 233 in the first direction 181 ismeasured from the second coupling surface 222 to the point of inflexionof the second coupling section 254, which is the point of inflexion ofthe triangle. The second length 223 in the first direction 181 ismeasured from the first coupling surface 221 to the second couplingsurface 222.

As depicted in FIG. 2, the second length 223 is smaller than the firstlength 213 and the third length 233, respectively. At the same time, thefirst length 213 and the third length 233 are identical. The coveringelement 200 is formed such that the second sub-element 220 is arrangedin the middle of the covering element 200 in the first direction 181.Hence, the covering element 200 is a symmetric covering element 200. Inthis exemplary embodiment, the second sub-element 220 is made of anon-magnetic material. Hence, the second length 223 in the firstdirection 181 is sufficiently great to ensure that no magnetic flux mayleak between the first sub-element 210 and the third sub-element 230.

FIG. 3 shows a cross section of a covering element 300 according to anexemplary embodiment of the invention. The cross-section has a firstnormal 141 being parallel to the third direction 183 (shown in FIG. 1).

A first sub-element 310, a second sub-element 320 and a thirdsub-element 330 are arranged one adjacent to another in the firstdirection 181. The second sub-element 320 is arranged in-between thefirst sub-element 310 and the third sub-element 330 in the firstdirection 181. A first length 313 is the length of the first sub-element310 in the first direction 181. A second length 323 is the length of thesecond sub-element 320 and a third length 333 is the length of the thirdsub-element 330, each in the first direction 181.

The first sub-element 310 comprises a first coupling section 253 whichis formed identical to the first coupling section 253 in FIG. 2, i.e. inthe shape of a triangle. A second coupling section 254 is formed as atriangle as well. The first coupling section 253 and the second couplingsection 254 are axis-symmetric to the middle axis 285. A first couplingsurface 221 and a second coupling surface 222 are each formed straight,i.e. parallel to the second direction 182. Hence, the first couplingsurface 221 and the first coupling section 253 are of different shape.Furthermore, the second coupling surface 222 and the second couplingsection 254 are also of different shape.

A first length 313 of the first sub-element 310 is smaller than a secondlength 323 of the second sub-element 320, which is itself smaller than athird length 333 of the third sub-element 330. The covering element 300is formed such that the second sub-element 320 is arranged eccentricfrom the middle of the covering element 300 in the first direction 181.In FIG. 3, the second sub-element 320 is arranged on the left side ofthe cross-section. When the covering element 300 is arranged in-betweentwo adjacent teeth of the stator a magnetic flux linkage from the firstsub-element 310 via the first coupling section 253 to one stator toothis smaller than a magnetic flux linkage from the third sub-element 330via the second coupling section 254 to another tooth.

FIG. 4 shows a covering element 400 according to an exemplary embodimentof the present invention. The covering element 400 comprises a firstsub-element 410, a second sub-element 420 and a third sub-element 430,which are arranged one adjacent to another in the same order in thefirst direction 181. As may be seen in FIG. 4, arranged one adjacent toanother means that a bigger part of the second sub-element 420 isarranged in-between the first sub-element 410 and the third sub-element430, although the second sub-element comprises two first protrusions 415which extend from the second sub-element 420 to the first couplingsection 453. Additionally, the second sub-element 420 comprises twosecond protrusions 435 which extend from the second sub-element 420 tothe second coupling section 454.

There may be an enlarged contact area between the second sub-element 420and the first sub-element 410 and the third sub-element 430,respectively, due to the two first protrusions 415 and the two secondprotrusions 435. Hence, an interconnection of the second sub-element 420to the first sub-element 410 and the third sub-element 430,respectively, may be reinforced.

Each of the two first protrusions 415 extends in the first direction 181into the first sub-element 410 over a small portion in the thirddirection 183. Therefore, the bigger part of the first coupling section453 in the third direction consists of the first sub-element 410. In thethird direction 183, the one of the first protrusion 415 and one of thesecond protrusion 435 are alternatingly arranged.

Similarly, each of the first protrusions 415 extends over less than onethird of the first sub-element 410 in the second direction 182. Hence,the bigger part of the first coupling section 453 consists of the firstsub-element 410. The two second protrusions 435 are formed accordinglyto the two first protrusions 415.

The first coupling section 453 is formed with a rectangular firstconnecting protrusion 455 and the second coupling section 454 is formedwith a rectangular second connecting protrusion 456. The firstconnecting protrusion 455 and the second connecting protrusion 456 mayeach be coupled to a respective groove in a stator tooth.

Hence, a first coupling surface 421 is enlarged compared to the firstcoupling surface 121 as shown in FIG. 1. Similarly, the second couplingsurface 422 is enlarged compared to the second coupling surface 122 asshown in FIG. 1. Additionally, in the third direction 183 a width of thefirst protrusion 415 and the second protrusion 435 increases along thefirst direction. Corresponding grooves in the first sub-element 410 andthe third sub-element 430, respectively, are provided. Hence, adove-tail connection is provided between the second sub-element 420 andthe first sub-element 410 and the third sub-element 430, respectively.By this dove-tail connection the fixation of the second sub-element 420in-between the first sub-element 410 and the third sub-element 430 isincreased and reinforced.

FIG. 5 shows a stator 500 and a covering element 100 according to anexemplary embodiment of the invention.

The covering element 100 is formed according to the covering element 100as shown and described in more detail in FIG. 1. The covering element100 comprises a first sub-element 110, a second sub-element 120 and athird sub-element 130 arranged one adjacent to the other in the firstdirection 181. The first sub-element 110 comprises the first couplingsection 153 and the third sub-element 130 comprises the second couplingsection 154.

The first direction 181 extends parallel to the circumferentialdirection of the stator 500. The second direction 182 extends parallelto the radial direction of the stator 500 and the third direction 183extends parallel to the axial direction of the stator 500.

The stator 500 comprises a first tooth 561 and a second tooth 562 with aslot 565 in-between the first tooth 561 and the second tooth 562. Thefirst tooth 561 comprises a first groove 563 in its tip region and thesecond tooth 562 comprises a second groove 564 in its tip region. Thefirst coupling section 153 is a protrusion with a V-shape and the firstgroove 563 has a complementary V-shape. Similarly, the second couplingsection 154 is a protrusion with a V-shape and the second groove 564 hasa complementary V-shape. The covering element 100 is inserted in thestator 500 by moving the covering element 100 in an inserting direction580 being parallel to an axial direction of the stator and the thirddirection 183, respectively. When the covering element 100 is insertedinto the slot 565, the slot 565 is closed in the radial direction andsecond direction 182, respectively. Hence, when a coil (not shown inFIG. 5) is inserted into the slot 565, the coil is securely held inplace in the slot 565 by the covering element 100.

A length of the covering element 100 in the third direction 183corresponds to the axial length of the stator 500 (not drawn to scale inFIG. 5).

FIG. 6 shows a stator 600 with an inserted coil 671, 672 before mountinga covering element 100 according to embodiments of the presentinvention.

The stator 600 comprises a first tooth 661 with a first groove 663 inits tip region and a second tooth 662 with a second groove 664 in itstip region.

In a first mounting step, a first coil 671 and subsequently a secondcoil 672 are inserted into a slot 665 between the first tooth 661 andthe second tooth 662. A first groove 663 is formed in a tip region ofthe first tooth 661 such that a total width of the slot 665 is notnarrowed in the tip region. A second groove 664 is formed in the tipregion of the second tooth 662 accordingly. Hence, the first coil 671and the second coil 672 may be inserted into the slot 665 without usinga special tool.

FIG. 7 shows the stator 600 of FIG. 6 after mounting the coveringelement 100 according to embodiments of the invention.

The covering element 100 comprises the first sub-element 110 with thefirst coupling section 153, the second sub-element 120 and the thirdsub-element 130 with the second coupling section 154. The first couplingsection 153 is formed as a V-shaped protrusion and the first groove 663is formed as a corresponding V-shape. The second coupling section 154 isformed in a V-shaped protrusion just as the first coupling section 153,and the second groove 664 is formed in a corresponding V-shape. As shownin FIG. 7 the first coupling section 153 is press-fitted to the firstgroove 663 and the second coupling section 154 is press-fitted to thesecond groove 664, respectively. After inserting the covering element100, the covering element 100 closes the slot 665 and securely holds thefirst coil 671 and the second coil 672 in place.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of ‘a’ or‘an’ throughout this application does not exclude a plurality, and‘comprising’ does not exclude other steps or elements.

The claims are as follows:
 1. A covering element for covering a slotbetween two adjacent teeth of an electromagnetic machine, the coveringelement comprising: a first sub-element; a second sub-element; and athird sub-element; wherein the first sub-element, the secondsub-element, and the third sub-element are arranged adjacent to eachother in a first direction, such that the second sub-element is arrangedin-between the first sub-element and the third sub-element; wherein thesecond sub-element is made of a different material than the firstsub-element and the third sub-element; wherein a sum of a first lengthof the first sub-element in the first direction, a second length of thesecond sub-element in the first direction and a third length of thethird sub-element in the first direction is a total length in the firstdirection, further wherein a middle of the second length of the secondsub-element in the first direction is distanced in the first directionfrom a middle of the total length in the first direction.
 2. Thecovering element according to claim 1, wherein the first sub-element andthe third sub-element are configured for being each coupled to a tipregion of a tooth.
 3. The covering element according to claim 1, whereina first magnetic permeability of the first sub-element is higher than asecond magnetic permeability of the second sub-element.
 4. The coveringelement according to claim 1, wherein a third magnetic permeability ofthe third sub-element is higher than the second magnetic permeability ofthe second sub-element.
 5. The covering element according to claim 1,wherein the second sub-element is made of a non-magnetic material or asoft magnetic material, further wherein the first sub-element and thethird sub-element are made of a magnetic material or a soft magneticmaterial.
 6. The covering element according to claim 1, wherein a sum ofa first length of the first sub-element in the first direction, a secondlength of the second sub-element in the first direction and a thirdlength of the third sub-element in the first direction is a total lengthin the first direction, further wherein a middle of the second length ofthe second sub-element in the first direction coincides with a middle ofthe total length in the first direction.
 7. The covering elementaccording to claim 1, wherein the first sub-element, the secondsub-element and the third sub-element are formed as an integral part. 8.A stator for an electromagnetic machine, the stator comprising: at leasttwo teeth; and at least one covering element according to claim 1,wherein each of the at least two teeth comprises a tip region with agroove, wherein the at least one covering element is arranged in-betweenthe at least two teeth, wherein the first sub-element comprises a firstcoupling section, wherein the third sub-element comprises a secondcoupling section, wherein the first coupling section and the secondcoupling section are each coupled in the respective groove of the twoteeth.
 9. The stator according to claim 8, wherein the respective groovehas a shape being complementary to a shape of a first coupling sectionor a second coupling section.
 10. A method of forming a coveringelement, providing a first sub-element; providing a second sub-element;providing a third sub-element; and arranging the first sub-element, thesecond sub-element, and the third sub-element adjacent to each other inthe first direction, such that the second sub-element is arrangedin-between the first sub-element and the third sub-element; wherein thesecond sub-element is made of a different material than the firstsub-element and the third sub-element; wherein a sum of a first lengthof the first sub-element in the first direction, a second length of thesecond sub-element in the first direction and a third length of thethird sub-element in the first direction is a total length in the firstdirection, further wherein a middle of the second length of the secondsub-element in the first direction is distanced in the first directionfrom a middle of the total length in the first direction.
 11. The methodaccording to claim 10, wherein the first sub-element and the secondsub-element are made as one integral part.
 12. The method according toclaim 11, wherein the first sub-element and the second sub-element aremade by pressing and heat treatment of a powder with different magneticproperties.